This invention relates to apparatus for grinding materials, and, more specifically, to a pulverizing bowl mill equipped with improved air deflector means that is particularly suited to be utilized for effecting the pulverization of coal.
It has long been known in the prior art to provide apparatus employable for purposes of effecting the grinding of materials. More specifically, the prior art is replete with examples of various types of apparatus that have been used heretofore to effect the grinding of a multiplicity of different kinds of materials. In this regard, in many instances discernible differences of a structural nature can be found to exist between individual ones of the aforesaid apparatus. The existence of such differences is, in turn, attributable for the most part to the diverse functional requirements that are associated with the individual applications in which such apparatus are designed to be employed. For instance, in the selection of the particular type of apparatus that is to be utilized for a specific application, one of the principal factors to which consideration must be given is that of the nature of the material that is to be ground in the apparatus. Coal is one such material wherein there is a need to grind it in order to render it suitable for use in certain applications. Furthermore, fossil fuel fired power generation systems represent one such application in which it is desired to employ coal, as the source of fuel therefor, and wherein a requirement exists to grind, i.e., pulverize the coal to render it suitable for use for this purpose.
To this end, coal has long been one of this nation's most abundant sources of fuel. At one time earlier in this century, much of the nation's energy needs were being met through the use of coal. Then, a decline set in in the degree to which coal was being employed to generate power. Much of this decline stemmed from the increased usage of oil and gas as sources of fuel. More recently, the power being generated from the burning of oil and gas has been supplemented by the use of nuclear fuel for power producing purposes. However, with the advent of the oil embargo earlier in this decade, which was accompanied by sharp increases in the price of oil and the existence of restricted oil supplies, and the increased concern, which has since been expressed over the rate at which the world's known oil reserves are being depleted, coal has begun to regain some of the favor which it once had as a source of fuel to meet the nation's energy needs. To some extent, this has been evidenced in the number of orders which have been placed in recent years for power generation systems that are to be coal fired as well as the extent to which increased interest is being shown in effecting the conversion of existing oil- and gas-fired power generation systems to coal-fired systems.
For purposes of the discussion that follows, the coal-fired systems referred to above are considered to consist of essentially the following major operating components: A coal feeder, apparatus for pulverizing coal, a distribution system for distributing the coal after the pulverization thereof, a furnace in which the coal is to be burned and the requisite controls for effecting the proper operation of the coal-fired power generation system. Of particular interest herein is that portion of the coal-fired system which has been identified above as the apparatus for pulverizing the coal. Coal pulverizing apparatus are not new. They have been known to exist in the prior art for more than half a century. Furthermore, many improvements in the construction and/or mode of operation of coal pulverizing apparatus have been made during this period.
There are a number of features that it is advantageous for any coal pulverizing apparatus to possess, but particularly those which are designed for employment in a coal-fired power generation system. Reference is had here to features such as reliability, low power consumption, minimum maintenance and wide range of capacity. In addition, such apparatus advantageously should also be characterized by quiet operation, integrated lubrication systems, convenient adjustment and control of coal flow and fineness, and the ability to handle the high temperature air that is required for high moisture coal.
One particular type of coal pulverizing apparatus which is to be found in the prior art that is advantageously characterized by the embodiment therein of the above-recited features is an apparatus most commonly referred to in the industry by the name bowl mill. The latter apparatus obtains its name by virtue of the fact that the pulverization, i.e., grinding, of the coal which takes place therein is effected on a grinding surface that in configuration bears a resemblance to a bowl.
Reference may be had, by way of exemplification, to U.S. Pat. No. 3,465,971, the latter being assigned to the same assignee as the instant application, for a teaching of the nature of the construction and the mode of operation of a prior art form of bowl mill that is suitable for use in a coal-fired power generation system to effectuate the pulverization of the coal that is to be burned as fuel therein. As taught by the afore-referenced patent, a bowl mill essentially consists of a body portion in which a grinding table is mounted for rotation, a plurality of grinding rollers that coact with the grinding table to effect the grinding of coal interposed therebetween, coal supply means for feeding to the interior of the bowl mill the coal that is to be pulverized, and air supply means for supplying to the interior of the bowl mill the air required in the operation of the latter. In accordance with the mode of operation of such a bowl mill, the coal, which enters the bowl mill, is pulverized by virtue of the coaction of the grinding rollers with the grinding table. After being pulverized, the coal particles are thrown outwardly by centrifugal force whereby the particles are fed into a stream of air that is entering the bowl mill. The stream of air, which now contains pulverized coal particles, flows through a tortuous path that is established in part by the positioning within the bowl mill of a suitably supported deflector means. As the stream of air and coal particles flows along the aforementioned tortuous path, the sharp turns contained therein effects the separation of the coarse coal particles from the air stream. These coarse coal particles are then suitably returned to the grinding table for further pulverization, while the fine coal particles are carried through the bowl mill in the air stream, and exit therefrom along with the air.
In a conventional coal-fired power generation system, a multiplicity of bowl mills of the type shown in the afore-referenced patent would commonly be employed for purposes of satisfying the requirements of the system for pulverized coal. By way of example, the capacity of each of the individual bowl mills might be on the order of one hundred tons per hour of coal.
Although bowl mills constructed in accordance with the teachings of the afore-referenced patent have, under actual operating conditions, proven capable of providing adequate performance to date, a need has nevertheless been evidenced for improvements to be made therein. More specifically, prolonged operation of this type of bowl mill has revealed the existence of several conditions of an undesirable nature that can arise during the use thereof. One of these stems from the fact that a space exists between the outer surface of the deflector means and the inner surface of the separator body, which serves to create an opening therebetween. During operation of the bowl mill, it has been found that coal particles tend to flow through this opening and/or accumulate therein, which, in turn, can adversely affect the performance desired from the bowl mill. Attempts to close these openings have only had the affect of tending to increase the susceptibility of coal particles to accumulate at such locations. Another disadvantage of a bowl mill equipped with deflector means of the type illustrated in the afore-referenced patent is that the deflector means is incapable of encompassing all of the air flow. The failure to do so produces poor circulation of air through the bowl mill as well as mill deterioration. A need has, therefore, been evidenced for a bowl mill embodying improved air deflector means, the latter being characterized by the following capabilities: The ability to encompsss all of the air flow, the ability to deflect the circulating air over the grinding table such that all air flow is diverted from the internal components of the bowl mill, the ability to supply a uniform layer of coal to each grinding roll, and the ability to inhibit the creation of pockets of stagnant air, particularly in the areas adjacent to the grinding rolls and/or the grinding table.
It is, therefore, an object of the present invention to provide a new and improved bowl mill suitable for use particularly in effecting the pulverization of coal.
It is another object of the present invention to provide such a bowl mill embodying improved air deflector means whereby the bowl mill equipped therewith is particularly suited for use in coal-fired power generation systems that require large quantities of pulverized coal.
It is still another object of the present invention to provide such a bowl mill having improved air deflector means that is operable to encompass all of the air flow entering the bowl mill.
A further object of the present invention is to provide such a bowl mill having improved air deflector means that is operable to deflect the circulating air over the grinding table such that all air flow is diverted from the internal components of the bowl mill.
A still further object of the present invention is to provide such a bowl mill having improved air deflector means that is operable to supply a uniform layer of coal to each grinding roll.
Yet another object of the present invention is to provide such a bowl mill having improved air deflector means that is operable to inhibit the creation of pockets of stagnant air, particularly in the areas surrounding the grinding rolls and/or the grinding table.
Yet still another object of the present invention is to provide such a bowl mill having improved air deflector means that is advantageously characterized by ease of manufacture and installation as well as by its relatively long life.